"Cosplay" is the contraction of "costume play",
and it's a sort of particular "carnival" during japanimation
conventions, where people disguise like a hero of manga/anime/comic/videogame/visual
group of their choice, and they have to look like their model the most
they can, with a quality costume totally home made.
This activity appared in France in 1996 and comes directly from Japan.

There's two kind of cosplay: free cosplay and competition cosplay. In
the free one, you simply roams in the conventions' corridors, for fun.
In the second case, you have to join a cosplay competition. You can
be alone or in a group, and you have to play a scene (copy known scenes,
sing, mime, execute choreographies with music...) in front of a very
enthusiastic public. There's generally prizes to win.